Growing up I was always teased, even tormented that I threw a ball like a girl. I find out now at sixty-three that you can be cured of this dreaded disability. But therapy has to start while the poor afflicted child is still young, the younger the better, to late for me.
"Throwing Like a Girl"
Apparently, throwing a ball is a learned complex skill that is ingrained into your central nervous system and is difficult to alter after childhood, so it's important your son learns how to throw the ball the correct way, like a man.
The way it works is emulation. Monkey see, monkey do. First, make sure you, yourself throw a ball like a man, then have the child watch you throw the ball.
Children find this fascinating and want to throw it themselves. Just like you. So, make sure you throw like a man, I can't stress the importance of this point.
To amplify matters, I've heard of this trick - Get a plastic wading pool for children. Set it up so you throw into it, so it makes a large sound as it bangs into it, it also gives you a target. The sound gets amplified, the harder you throw it, which encourages the kid to throw it harder, associating loudness with throwing harder.
Say, you have a girl, and want her throw a ball like a girl, get someone who does throw like a girl. Let the kid watch that person throw the ball. The kid will throw just like her in short time.
Throwing a ball is a learned ability. Catching a ball is a skill. Skills can be taught throughout life, abilities cannot, so start young.